Bilirubin is a principal component of bile pigment in body fluid. Bilirubin present in serum is a product of the decomposition of heme originating from hemoglobin in red blood cells. Two fractions of bilirubin are present in blood serum, unconjugated and conjugated bilirubin. Conjugated bilirubin is bilirubin which is conjugated with glucuronic acid in the liver and rendered water soluble. The conjugated bilirubin also is referred to as "direct" bilirubin, and the bilirubin not conjugated with glucuronic acid is referred to as "indirect" bilirubin or unconjugated bilirubin. Normally, only small amounts of bilirubin are found in the blood, the normal concentration being for direct bilirubin up to about 0.25 mg/l 100 ml serum (.ltoreq.4.3 .mu.mol/L); and for indirect bilirubin up to about 0.75 mg/100 ml serum (.ltoreq.12.7 .mu.mol/L). The content of bilirubin in blood increases with an increase in decomposition of hemoglobin and a decrease in liver function.
The determination of both bilirubin fractions is of importance in medical diagnosis. Normally, bilirubin is excreted from the gall bladder with the biliary fluid into the intestine. However, this mechanism is disturbed in various disease states. Thus, for example, in the case of increased hemoglobin breakdown, the bilirubin conjugation system can be overloaded so that the ratio of direct/indirect bilirubin is changed. In the case of liver cell damage, disturbances of the outflow in the biliary capillary or bile duct obstructions, the excretion of the bilirubin via the gall bladder into the intestine is reduced or completely blocked. This leads to increased bilirubin concentrations in the blood. The absolute concentration of the bilirubin and the ratio of direct/indirect bilirubin can thereby be influenced. Thus, from the measurement of both values, important diagnostic conclusions can be made regarding the nature and localization of certain diseases of the liver, gall bladder and intestinal tract. Generally, the total bilirubin is usually determined first, and then the direct bilirubin content is measured. The indirect bilirubin portion is obtained from the difference between the two values. Methods for measuring serum bilirubin are reviewed in Doumas and Wu, Critical Reviews in Clinical Laboratory Sciences, 28:415-445 (1991); and Lott and Doumas, Clin Chem., 39:641-647 (1993), the disclosures of which are incorporated herein.
Analytical tests which permit the quantitative analysis of bilirubin are very useful clinically. The most widely used assay for bilirubin has been the so called diazo method. In the diazo method, a sample suspected of containing bilirubin is contacted with a reagent composition which includes a diazonium salt. The diazonium salt reacts with bilirubin to form two azobilirubin fragments. The azobilirubin has an extinction coefficient which is higher than that of bilirubin itself and is easily detectable.
Many diazonium salts have been used in the diazo method for determining bilirubin. For example, diazotized sulfanilic acid couples with bilirubin to give a yellow diazobilirubin pigment. Details of the diazo method for quantitative analysis of bilirubin are described in Doumas et al., Clin Chem., 31:1779-1789 (1985); M. Michaelsson, Scand. J. Clin. Lab. Invest., 13 (Suppl.), 1-80 (1961); H. Malloy, J. Biol, Chem., 119, 481(1939); and Z. K. Shihabi, et al., American Journal of Medical Technology, 43(10), 1004-1007 (1977), the disclosures of which are incorporated herein. Other diazonium salts, such as 2,4- and 2,5-dichlorophenyldiazonium salts, have been used for the detection of bilirubin in serum and urine. However, methods using these diazonium salts are known to be relatively insensitive, and some of these diazonium salts, when dry, are explosively unstable, i.e., subject to shock induced decomposition. U.S. Pat. No. 4,468,467 to Babb et al. Another diazonium compound which has been used for the determination of bilirubin is diazotized sulfanilamide. Chin-Chung Chen et al., Clin Chem., 26:990 (1980). Synermed.TM. (Synermed, Inc., Quebec, Canada) total bilirubin reagent is commercially available which includes a stabilized diazonium salt of 3,5-dichloroaniline, 3,5-dichlorophenyl diazonium tetrafluoroborate, which reacts with bilirubin to form azobilirubin which absorbs maximally at 540 nm. The red azobilirubin formed can be shifted to a blue color which absorbs at 600 nm upon the addition of alkali. Caffeine and surfactants are used as reaction accelerators.
U.S. Pat. No. 4,468,467 to Babb et al. describes certain substituted sulfanilamide and carbonamide diazonium salts for bilirubin assays. U.S. Pat. No. 4,902,477 to Katsuyama et al. discloses an analytical element for quantitative analysis of bilirubin by a diazo method, which includes certain aryldiazonium salts having in the aryl group a substituent which is an alkoxycarbonyl group, an alkylaminosulfonyl group or an alkylaminocarbonyl group. U.S. Pat. No. 4,892,833 to Weiss et al. discloses aryldiazonium salts wherein the aryl group is substituted with halogen groups and lower alkoxy groups for use in the determination of bilirubin.
Many of the assay reagents used in current assays have problems associated with their use. For example, in the case of automated bilirubin assays using a commercially available dichloroaniline diazonium tetrafluoroborate salt, the diazonium form of the chloroaniline derivatives is known to interact strongly with indican, a compound found in the serum of renal dialysis patients making it unsuitable for this sample type. Hemolyzed samples also cannot be used in many of the assays due to extensive interference by hemoglobin. Some of the reagents have a relatively low solubility in aqueous systems, thus reducing their usefulness in these systems. Additionally, many of the reagents are unstable in the liquid form, cannot be readily transported, and do not have a useful shelf-life.
It is an object of the invention to provide diazonium ion compounds which can be used as assay reagents for the detection of bilirubin in body fluid samples. It is a further object of the invention to provide diazonium ion compounds which are thermally stable making long distance shipping more feasible. It is yet another object of the invention to provide diazonium ion compounds which are thermally stable, have a long term shelf-life, and may be stored while retaining activity, for example, for periods of one year or more.